External combustion engines are known, also known as Stirling engines, which exploit a difference in temperature caused in a thermodynamic fluid and actuate the cyclical and alternate movement of a displacer and a piston.
In particular Stirling engines are known of the so-called gamma type, which comprise a first cylinder and a second cylinder disposed in quadrature with respect to each other, that is, with their respective axes angled by 90° to each other, and in which a first piston, also called displacer, and a second piston slide. The displacer and the second piston are connected by means of respective connecting rods near a single crank pin. The latter is keyed onto a drive shaft from which the mechanical work obtained is taken.
The first cylinder is provided with a hot part disposed near the head, or in other words, near the upper dead point of the displacer, and with a cold part disposed near the lower dead point of the displacer. The hot part and the cold part of the first cylinder are respectively heated and cooled to transfer heat to the thermodynamic fluid contained in the first cylinder.
The hot part and the cold part of the first cylinder are suitably connected fluidically with each other, for example by providing bleeding between the external jacket of the first cylinder and the displacer.
The first cylinder, near its cold part, is provided with a pipe connecting with the head of the second cylinder, so as to create a fluidic connection between the first and second cylinder.
By exploiting the expansion of the thermodynamic fluid due to the contribution of heat from the hot part, the second piston moves toward its lower dead point. The displacer moves toward the cold part, entailing a cooling of the previously heated thermodynamic fluid and therefore entailing a contraction of the fluid, which draws the second piston toward its upper dead point.
The alternate movement of the second piston from the upper dead point to the lower dead point causes the drive shaft to rotate and hence the mechanical work to be generated.
Although this type of engine is silent, has a low environmental impact and requires limited maintenance, it does not allow variations and modulations of the nominal power, and substantially functions always at the same capacity.
Due to this limitation, such engines are almost exclusively used in applications where a continuous and constant delivery of energy is required.
In order to increase the flexibility of this type of engine, the international patent application WO-A-2010/070428 is known, in the name of the present Applicant, which provides the possibility of varying the reciprocal angle between the first cylinder and the second cylinder in order to vary the cc volume of the engine and hence to vary the functioning modes or the rotation speed of the engine itself.
Although this solution allows to vary the rotation speed of the engine and hence to adapt it to functioning requirements almost instantaneously, as requested by the user, it is in any case more complex than a static engine, and also, in particular configurations, it may have a rather low functioning performance, which is more accentuated the more the first and second cylinder are distanced from their quadrature condition. The reduced efficiency is determined by the increase in idle volumes in the first and second cylinder, that is, fluid that expands/compresses and does not generate any useful work. In certain situations, this can lead to this type of engines with variable configuration being abandoned if the applications require a substantially constant supply of energy.
In fact, in order to satisfy certain requirements and requests, in particular for less complexity and a lower economic cost, it is necessary to achieve static engines, and therefore not with a variable configuration of the reciprocal angle between the first and second cylinder, which have a good performance and are not bulky.
On the contrary, the gamma type engine, given the disposition of the first and second cylinder, has a very bulky engine which in particular applications is not acceptable.
The same document WO-A-2010/070428 also describes a form of embodiment of the external combustion engine in which kinematic connection means, such as connecting rods and bars, are associated to the first and second piston, and respectively to crank means configured to rotate around an axis of rotation and move the first and second piston in an alternate motion.
The crank means comprise a first crank pin and a second crank pin disposed angularly offset with respect to each other and at the same distance from the axis of rotation, so as to achieve a travel of the first piston that is identical to that of the second piston.
This form of embodiment of the engine, in some particular applications, may be not very efficient from the thermodynamic point of view, given that the heat exchanges of the work fluid are not optimized in the first and second cylinder.
Indeed it is known that the mechanical work absorbed during the expansion and compression of a hot fluid, given the same variation in volume to which it is subjected, is always greater than the mechanical work absorbed during the expansion and compression of a colder fluid. In consideration of this, the engine described in the state of the art does not allow to optimize the relation between the heat exchanges in the hot and cold part, and the functioning kinematics of the engine.
Purpose of the present invention is to obtain an external combustion engine which is compact, simple to make, efficient and economical.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.